1. Field of the Invention
This invention relates generally to high power semiconductor devices, and more specifically, to high power MOS devices preferably in discrete form and fabrication methods therefor.
2. Description of the Prior Art
In the past, high power semiconductor devices were generally fabricated as bipolar discrete devices. Generally, these high power bipolar discrete devices were customarily NPN type transistors although some high power devices were made using PNP type transistors or thyristors which were either of NPNP or PNPN type. In addition, for some applications, high power semiconductor diodes of either NP or PN type were also fabricated and used as high power devices.
However, as the semiconductor technology developed, people in the industry recognized a need to develop reliable, high power, MOS type semiconductor devices especially because of the use of fewer fabrication steps and generally simpler design. One advantage of a metal-oxide-silicon (MOS) semiconductor device as contrasted with a bipolar semiconductor device is that generally less diffusion steps are required to make the MOS device and, consequently, it is generally less costly to produce large volumes of these types of devices. MOS type semiconductor devices are basically field effect transistor devices and considered as "unipolar" type devices as contrasted with the "bipolar" conventional semiconductor transistor devices. Furthermore, high power bipolar discrete devices have become fairly complex because of the use, in many cases, of extra features or steps such as diffused guard rings in the semiconductor substrate surface and field relief electrodes located on the insulating surface of the bipolar discrete device.
Recently, in various development programs created to produce high power discrete MOS type semiconductor devices, there has been very serious device design and manufacturing problems associated with reliably producing in heavy volume, high power MOS discrete devices.
One of the problems associated with making a reliable, high power, MOS type discrete device has been the need to develop a MOS semiconductor structure which would have a design that would permit the formation of a large metal heat sink coating that would cover substantially one entire surface portion of the MOS semiconductor device to permit large amounts of current to be carried by the high power, MOS device.
Another problem associated with the development of high power, MOS discrete semiconductor devices is to reliably produce large quantities of these devices with high device yields which would greatly reduce the cost for these types of devices. This has become a very serious problem because it became apparent to developers of these high power MOS devices that conventional, planar type gate electrode structures could not be used. In order to form a high power or high current carrying MOS discrete device, it is necessary to understand that the current of any MOS device (I.sub.D) is directly proportional to the value Z which is the longitudinal dimension of the channel region between the source and drain regions of the MOS device and is inversely proportional to the dimension L which is the transverse distance between the source and drain regions of the MOS device.
The dimension L cannot be made smaller than about 3 microns due to both the problem created by photoresist limitations and because of the problem associated with the relatively unpredictable extent of sideways diffusions when a conventional source and drain diffused region is formed in a standard MOS device design. However, the semiconductor technology has developed an alternate way to produce more precise and accurate differences using certain other diffused regions such as the very narrow base widths that have been formed between emitter and collector regions of a bipolar transistor structure after emitter and base diffusions. Thus, vertical differences obtained by using diffused regions in a vertical direction can be made as small as about 1 micron. Accordingly, development of MOS structures using this 1 micron difference in a vertical dimension to form a gate region therein has recently become important to close the transverse spacing between source and drain regions thereby reducing the L of the FET structure which increases the device current I.sub.D.
One type of MOS device that has been made in the past in an attempt to further improve MOS structures is a D-MOS device. A D-MOS device is fabricated by performing sequential diffusions in the same opening in the silicon dioxide. The channel length L is thus the difference between these two diffusions. D-MOS devices offer advantages over conventional MOS devices, but the current density is still too small for a power MOSFET.
In order to increase the Z of the MOS device which would thereby create a direct proportional increase in the current I.sub.D that can be carried by the device, it was found expedient to form a sinuous MOS structure in order to significantly increase the Z value of the device by having longer source and drain regions.
Consequently, in order to maximize Z and minimize L, it was deemed necessary to develop MOS structures that did not use conventional planar gates, the usual source and drain regions nor the standard contacts to source and drain regions. One very recent MOS type structure that was developed used a V-shaped groove through a diffused heavily doped region (which functioned as the source or drain region after being split by the V-shaped groove); however, the metal gate electrode (usually of aluminum) formed on an insulating layer located on the V-shaped groove was not reliable due to the great difficulty in providing a continuous metal layer onto the V-shaped groove's insulation layer surface. Tiny cracks across the metal layer formed in the recess portion of the V-shaped groove prevented the metal gate electrode from performing its usual channel formation function between source and drain regions.
Accordingly, the need existed to develop very reliable, high power, MOS discrete devices which would have the significant advantage of high current carrying capacity and reliable V-shaped gate electrodes